This invention is directed to selective hydrogenation of dienes to monoolefins, particularly of cyclopentadiene to cyclopentene. More specifically, it is directed to a process where cyclopentadiene is selectively hydrogenated to cyclopentene in which a mixture of cyclopentadiene and hydrogen are passed through a catalyst comprising a highly dispersed nickel which has been saturated with ammonia.
At the present time substantial amounts of cyclopentadiene usually in the form of dicyclopentadiene are available as a byproduct from the process of steam cracking of naphtha in order to produce ethylene. Cyclopentene has been found to be useful as a monomer for the formation of general purpose elastomers by ring opening polymerization. Therefore, it is desirable to convert a portion of the excess cyclopentadiene available into a more valuable raw material, cyclopentene.
The hydrogenation of cyclopentadiene to cyclopentene is not new. In U.S. Pat. No. 3,751,499, cyclopentadiene has been continuously converted to cyclopentene using a palladium on alumina catalyst. In U.S. Pat. Nos. 3,472,763, 3,915,891 and 3,819,734 further describe the conversion of cyclopentadiene into cyclopentene by the use of nickel salts as the catalyst. Other U.S. patents which relate to the non-continuous preparation of cyclopentene from cyclopentadiene are 3,857,894; 3,994,986; and 2,360,555.
However, none of these patents disclose the invention hereinafter described.
The cyclopentadiene employed to form cyclopentene by the use of the present invention is usually obtained by depolymerizing or cracking of dicyclopentadiene. The depolymerization of dicyclopentadiene is usually accomplished by heating the dimer at a temperature above 150.degree. C. under atmospheric pressure in a conventional cracking apparatus. The depolymerized cyclopentadiene should be employed or hydrogenated without substantial delay because it will redimerize on long standing at ambient temperature.